Venus is too hot and Mars is freezing cold. Earth is just right. Have you ever wondered why? Yes Earth is just at the right distance from the sun but there is one major factor, which has created and sustained life on earth. It is the greenhouse effect, a process by which the natural gases present in the Earth's atmosphere prevent the escape of heat or energy from Earth thereby making it habitable. In short Greenhouse effect keeps the temperature of the Earth in the range where life can flourish.
Have you ever been to a greenhouse? Did you notice that the temperature inside is few degrees higher than out side? This happens because the glass traps some of the heat from the sunlight. Same thing happens inside a car on a sunny day. The car windows allow the sun-rays to heat up the car, but prevent much of the heat escaping. That is why the inside of your car is warmer on sunny days. Now imagine this effect on a magnanimous scale and you will begin to get a clearer picture.
Lets understand the Science behind the Greenhouse effect. The sun radiates vast quantities of energy into space, across a wide spectrum of wavelengths. About 49 - 50% of the sunlight has wavelengths in the range of 700-1000nm, which is the infrared region. Some of the infrared radiation is absorbed by earth and some is reflected back to the space. About 7-8% of Sunlight is ultraviolet light (wavelength lesser than 400 nm), which causes skin damage and sunburn, as it is capable of breaking biological molecules. About 99% of the Ultraviolet light is absorbed by the Ozone layer, thus protecting us from potential damage of these high frequency rays.
The remaining 43% of this energy is in the visible (400 and 700 nm wavelength
Our atmosphere is pretty transparent to visible light so this part of the sunlight comes in straight to us. Some of it is absorbed by the earth (Land, Oceans and Vegetation) in daytime and emitted back into the atmosphere in the form of infrared radiation at night. Logically the earth should then cool down very rapidly at night, but that doesn't happen as Earth's atmosphere contains some gases, which absorb, emit and reabsorb this energy among themselves, thus preventing most of the heat from escaping into the space. These gases are called 'Greenhouse gases' because they hold heat in like the glass walls of a greenhouse.
Water vapor (H20), carbon dioxide (CO2), Methane (CH4) and Nitrous oxide (N2O) are all natural greenhouse gases. These molecules are composed of more than two atoms, which are bound loosely together. Low energy from the infrared rays can cause these molecules to vibrate. The vibrating molecule emits the radiation again which might be absorbed by another greenhouse gas. This phenomenon traps the heat near the surface of the earth thus providing us the much-needed blanket for survival. The Greenhouse Effect was discovered by Joseph Fourier in 1827.Human activity in the last 200 years has increased the amount of some of the greenhouse gases thereby making this blanket hotter. Humans have not only increased the amounts of these natural greenhouse gases but have also created some artificial ones like Chlorofluorocarbons (CFC),which have started to alter the delicate balance in the atmosphere. CFC's are now banned from being released into the atmosphere, but continuous burning of fossil fuels, cutting of precious tropical forests, mining and other man-made activities are causing changes slowly and it's beginning to show up. This has lead to much of hue and cry about Global warming and the adverse effect it could have on our future as a living planet.